Electronic instruments which employ a separate tone generator for each note within the range of the instrument tend to be expensive because a large number of tone generators are required; for example, there are sixty-one notes within the musical range of a typical electronic organ. Accordingly, in recent years other tone generation techniques have been adopted which produce the same number of notes but require a much smaller number of tone generators. All of these techniques involve the generation of a limited number (one or more) of high frequencies from which all the necessary lower frequencies are obtained by dividing down. Because of this derivative relationship between the low frequencies and the higher ones, they are necessarily synchronous in phase. This phase synchronism causes a problem whenever octavely related notes are played simultaneously: the resulting chord has a thin, dry sound, unlike the full chorus effect which is characteristic of an acoustic instrument and also of an expensive electronic instrument having an individual tone generator for each note.
There are a number of different versions of the frequency division technique for lower tone generation, and they all suffer from this problem. One variation is to employ twelve chromatic notes of the highest octave. Then the corresponding notes of all the lower octaves are obtained by twelve respective frequency division flip-flop chains proceeding in octave steps (i.e. each step involves division by two). Now that digital techniques and integrated circuit chips are widely used in electronic musical instruments, an even more economical variation of this technique employs a top-octave synthesizer chip driven by a single high frequency clock source to generate the twelve highest notes.
There is another variation of the frequency division approach which, like the top-octave synthesizer approach, also requires only one high frequency clock source. In this method, a limited number of variable divisor frequency dividers are used on a time-sharing basis. Only as many frequency dividers are used as are necessary to cover the maximum number of notes which a player will ask the instrument to play simultaneously, e.g. ten to twelve. Each frequency divider can be made to generate any one of the sixty-one notes in the musical range of the instrument, by specifying the proper divisor.
Each of these approaches tends to produce dry-sounding octave chords, because of the phase synchronization between any two octavely related notes. In the time-sharing system, for example, regardless of which divisors are employed at any moment, each frequency divider derives its high frequency signal from the same source as every other frequency divider does. If any two of these dividers are playing octavely related notes simultaneously, the ratio between their output tone frequencies will be exactly 2:1. Whenever two signals are sub-multiples of the same source, and their frequencies are in any integral ratio (such as 2:1), their waveform will be synchronized in phase, producing an undesirable thin, dry sound. The top-octave design approach also suffers from this tendency, because every pair of octavely related notes is derived by dividing the same signal source, and they also are related in frequency by a 2:1 ratio exactly.
In contrast, acoustical instruments and electrinic instruments of the independent generator type produce a pleasing full chorus audio effect, even when octavely related notes are sounded simultaneously. This is because there is unlikely to be phase synchronism between the waveforms produced by any two randomly related acoustical or electronic sources. An additional reason is the probability that their frequency ratio will differ slightly from the 2:1 mathematically ideal value, even though they are nominally in an octave relationship.
In order to make an electronic instrument of the single clock source type sound more natural, U.S. Pat. No. 3,828,109 of Morez, in FIGS. 1 and 4, has suggested the following approach. The clock frequency is divided down in successive steps of two by a chain of flip-flops, and these 2:1 related outputs are applied to the respective inputs of individual octave synthesizer circuits, which then create the full chromatic scale (twelve notes) for respective individual octaves. But special means are provided for adding or subtracting small numbers of pulses at the inputs of all but one of the synthesizer circuits, so that their effective source frequency ratios are slightly detuned, i.e. they differ slightly from the nominal 2:1 value. One disadvantage of this approach is that it requires a multiplicity of octave synthesizers, one for each octave in the musical range of the instrument. In addition, this approach is not applicable to the time-shared frequency divider type of instrument.
Furthermore, in this arrangement the altered source frequency differs only from one octave to another octave, but not from one note to another note within an octave. Consequently, while each pair of octavely related notes has a frequency ratio slightly detuned from the ideal 2:1 value, the degree of detuning is exactly the same for each note within an octave; it cannot be made to vary from note to note. This lack of note-to-note variation is a disadvantage. The maximum chorus effect is achieved by not only slightly detuning each pair of octavely related notes, but by also varying the specific degree of detuning from note to note within an octave. Instead of merely preventing phase synchronization between octavely related pairs of notes, this guarantees that there will also be a difference in the degree of asynchronism for each such pair. These two effects together result in fuller sound than could be achieved by repetitious use of the same degree of asynchronism.
In FIG. 3 of the Morez patent, cited above, another prior art approach to artificial chorus effect generation is disclosed. A pair of duplicate octave synthesizers is used for generating each octave, with appropriate provision for adding or subtracting a small number of pulses at the input of only one of the synthesizers of each pair, so that their effective source frequencies are not exactly the same. Once again, this approach is not directly applicable to the environment of a time-shared, variable divisor type of instrument. In addition, it does not have any provision for varying the degree of chorus effect from one note to another within an octave, because the number of pulses added or subtracted is determined on a whole-octave basis (just as it is in the other Morez approach described earlier), rather than on a note-by-note basis.